Pumpjack units reciprocate a long string of sucker rod which in turn reciprocate a downhole pump. From time to time, for one reason or another, the pump will malfunction and place tension loads on the sucker rod string in excess of the breaking strength of the rod string. The overloaded rod string will part most anywhere between the polish rod and the downhole pump, wherever the weakest part of the rod string may be.
It is undesirable to subject a sucker rod string to excessive tension, whereby some weak point in the rod string parts, because the lower end of the rod string located below the break must be fished, which can be a very time consuming and expensive job. Furthermore, if the structural integrity of the entire rod string is sound, it follows that all of the parts of the entire string has been subjected to undue tension loads which renders subsequent use of the string questionable.
In recent years, steel sucker rods have sometime been replaced with fiberglass sucker rods. The fiberglass sucker rod string usually has exceedingly long joints which must be replaced in its entirety whenever one of the joints break, and for this reason, it is especially desirable to never subject the string of the fiberglass sucker rod to a tension force approaching the breaking strength of the individual joints.
It would therefore be desirable to provide a string of sucker rod with a safety shear tool which includes a shear member therein which fails and enables the tool to part whenever the sucker rod string is subjected to a predetermined tension which is less than the designed breaking strength of the rod string. A safety shear tool which achieves this purpose is the subject of the present invention.